This project is designed to provide information about the populations of motor units that make up large limb muscles in mammals. Recent work has concentrated on the electrophysiological and morphological characteristics of spinal cord motoneurons, particularly on neuroanatomic studies and computer modeling of individual, functionally-identified motoneurons. We have attempted to identify the fundamental factors that control dendritic morphology by developing a relatively simple stochastic (Monte Carlo) model that can reproduce a wide range of statistical properties of actual motoneuron dendrites. This approach is being used to compare the fundamental dendritic structure of several groups of cat motoneurons and interneurons, as well as the morphologies of dendrites in two groups of motoneurons during postnatal development. We are extending these studies to examine the quantitative characteristics of dendritic trees in three dimensions, in order to explore whether the 3D anatomy of dendrites results, at least in part, from factors intrinsic to the dendrites themselves. Our dendritic simulations take account only of intrinsic factors. These simulations and the data that underlie them are also being used to test ideas about whether dendritic morphologies are optimized for particular functions while minimizing factors that can be regarded as biological costs.